Fiber Bragg grating (FBG) sensors have been playing a vital role in many industrial applications owing to their high sensitivity, fast response, immunity to electromagnetic interference, high reliability, distributed and multiplexing capability, and multiparameter sensing. The past decade has seen a tremendous growth in the number of FBG-based sensor systems. Nowadays, temperature and strain measurement encompass a wide variety of needs and applications in scientific fields, aerospace, metallurgical and civil engineering, solar panels, nuclear power, shipping, petroleum, and thermal power industries.
A fiber Bragg grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber core, which generates a wavelength specific dielectric mirror. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.
Fiber Bragg Gratings are made by laterally exposing the core of a single-mode fiber to a periodic pattern of intense ultraviolet light. The exposure produces a permanent increase in the refractive index of the fiber's core, creating a fixed index modulation according to the exposure pattern. This fixed index modulation is called a grating. At each periodic refraction change a small amount of light is reflected. All the reflected light signals combine coherently to one large reflection at a particular wavelength when the grating period is approximately half the input light's wavelength. This is referred to as the Bragg condition, and the wavelength at which this reflection occurs is called the Bragg wavelength. Light signals at wavelengths other than the Bragg wavelength, which are not phase matched, are essentially transparent.
Sensing technologies based on optical fiber have several inherent advantages that make them attractive for a wide range of industrial sensing applications. They are typically small in size, passive, immune to electromagnetic interference, resistant to harsh environments and have a capability to perform distributed sensing. Because of their telecommunication origins, fiber optic-based sensors can be easily integrated into large scale optical networks and communications systems.
Although developed initially for the telecommunications industry in the late 1990's, fiber Bragg gratings (FBGs) are increasingly being used in sensing applications and are enjoying widespread acceptance and use. The FBG is an optical filtering device that reflects light of a specific wavelength and is present within the core of an optical fiber waveguide. The wavelength of light that is reflected depends on the spacing of a periodic variation or modulation of the refractive index that is present within the fiber core. This grating structure acts as a band-rejection optical filter passing all wavelengths of light that are not in resonance with it and reflecting wavelengths that satisfy the Bragg condition of the core index modulation. The Nobel Laureate Sir William Lawrence Bragg established the Bragg law in 1915, describing with a simple mathematical formula how X-Rays were diffracted from crystals. The Bragg condition, when applied to fiber Bragg gratings, states that the reflected wavelength of light from the grating is:λB=2·neff·ΛG 
where, neff is the effective refractive index seen by the light propagating down the fiber and ΛG is the period of the index modulation that makes up the grating.
Fiber Bragg gratings (FBGs) are widely used as sensing elements for the measurement of physical parameters such as strain, pressure and temperature. The variation of these parameters induces changes of the central Bragg wavelength. The precise measurement of this wavelength change is crucial for achieving good sensor performance.
Several interrogation techniques based on bulk filters, fiber edge filters, edge optical spectra, edge fiber grating spectra, edge detector spectral responses, tunable fiber filters, tunable acousto-optic filters, tunable single mode laser diodes, receiving FBGs, interferometric detection, fiber lasers and Fourier techniques have been proposed.
An FBG illuminated by a broadband light source reflects a particular narrow band wavelength called Bragg wavelength and transmits all others. The Bragg wavelength is mainly dependent on applied temperature and strain. In general, the Bragg wavelength shift of FBG is monitored by using an optical spectrum analyzer (OSA). However, OSA has its own limitations in response time, resolution, weight, size, and cost. To overcome these issues, different interrogation techniques have been developed.
A more recent advance is the use of a simple technique based on converting the wavelength information into its equivalent intensity modulated signal which can be measured by using a photodiode with simple electronics. However, the inventor of the present disclosure has recognized the need for a more robust, low cost FBG sensor which exploits this technique.
This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.